A conventional vacuum servo apparatus generally includes:
a housing formed with at least a pressure space at an inner portion thereof, PA1 a movable wall installed in the housing such that the movable wall is capable of progressing and regressing relative to the housing and dividing the pressure space into a front chamber communicating with a negative pressure source and a rear chamber selectively communicating with the front chamber and the atmosphere, PA1 a power piston coupled to the movable wall, PA1 an input member arranged at an inner portion of the power piston progressably and regressably relative to the power piston and movable by operating an operating member, PA1 a valve mechanism having a negative pressure valve communicating the rear chamber with the front chamber in accordance with movement of the input member and an atmospheric valve communicating the rear chamber to the atmosphere in accordance with the movement of the input member, PA1 an output member for outputting a progressive force of the power piston to outside of the device by being made to progress by progress of the power piston in accordance with movement of the movable wall, and PA1 a reaction force member for transmitting the progressive force of the power piston and an input applied on the input member to the output member and exerting a reaction force having a magnitude in correspondence with an output from the output member to make the input member regress by being elastically deformed to bulge in a rear direction, and PA1 in which an elastic deformation amount of the reaction force member is increased or decreased in accordance with at least a change in a temperature of the reaction force member and the atmospheric valve is closed by making the input member regress by receiving the reaction force from the reaction force member.
According to the conventional vacuum servo apparatus, when the input member is made to progress by operating the operating member by the input of a driver, the negative pressure valve of the valve mechanism is closed, communication between the front chamber and the rear chamber is cut, the atmospheric valve of the valve mechanism is opened and the rear chamber is made to communicate with the atmosphere. By communicating the rear chamber with the atmosphere, pressure difference is produced between the front chamber and the rear chamber, the progressive force is produced in the movable wall and the power piston and the output member outputs the progressive force of the power piston to outside of the device. When the output member outputs the progressive force of the power piston, the reaction force member exerts the reaction force having the magnitude in correspondence with the output from the output member to the input member to thereby make the input member regress and the input member is made to regress by which the atmospheric valve is closed and communication between the rear chamber and the atmosphere is cut. That is, according to the valve mechanism, there is constituted a balanced state in which the atmospheric valve and the negative pressure valve are brought into a closed state and the vacuum servo apparatus generates the output in correspondence with the input to the operating member.
However, according to the conventional vacuum servo apparatus, easiness of elastic deformation of the reaction force member is changed in accordance with a change in the temperature of the reaction force member and the elastic deformation amount is increased or decreased and therefore, there is a concern that an input and output characteristic of the vacuum servo apparatus is varied in accordance with the temperature change of the reaction force member.
U.S. Pat. No. 5,483,866 discloses a vacuum servo apparatus that includes a housing forming at least one pressure space, a movable wall installed in the housing for dividing the pressure space into a front chamber communicated with a negative pressure source and a rear chamber selectively communicated with the front chamber and the atmosphere, a power piston coupled to the movable wall, an input member arranged at the inner portion of the power piston for moving relative to the power piston, a valve plunger member arranged in the power piston for moving integrally with the input member, and a control valve having an atmospheric valve seat arranged at the valve plunger member, a negative pressure valve seat arranged in the power piston, an atmospheric seal portion attachable and detachable to and from the atmospheric valve seat for cutting off communication between the rear chamber and the atmosphere by being brought into contact with the atmospheric valve seat and communicating the rear chamber with the atmosphere by being separated from the atmospheric valve seat, and a negative pressure seal portion attachable and detachable to and from the negative pressure valve seat for cutting off communication between the front chamber and the rear chamber by being brought into contact with the negative pressure valve seat and communicating the rear chamber with the front chamber by being separated from the negative pressure valve seat. The apparatus also includes an output member for outputting a progressive force of the power piston in accordance with movement of the movable wall, a reaction force member for transmitting the progressive force of the power piston and an input applied to the input member to the output member and providing a reaction force having a magnitude corresponding to the output from the output member to make the input member regress, and an actuator for making the rear chamber and the atmosphere communicatable to each other by separating the atmospheric valve seat from the atmospheric seal portion by forwardly moving the valve plunger member.
The conventional vacuum servo apparatus separates the atmospheric valve seat from the atmospheric seal portion by moving the valve plunger member through operation of the actuator separately from operation of the input member and produces a pressure difference between the front chamber and the rear chamber by communicating the rear chamber with the atmosphere. When the pressure difference is produced between the front chamber and the rear chamber by operating the actuator, progressive forces are produced at the movable wall and the power piston and the output member outputs the progressive force of the power piston to outside of the device.
When the vehicle is brought into the oversteered state in turning the vehicle, in the brake hydraulic pressure device of the vehicle having the brake steering control (oversteering restraining control) for supplying the brake fluid to the outer side front wheel of turning by operating the pump without braking operation of the driver and temporarily increasing the brake hydraulic pressure of the outer side front wheel of turning to thereby escape from the oversteered state, the prestage pressurizing function is requested for supplying the pressurized brake fluid to the pump such that the pump can supply the brake fluid smoothly to the outer side front wheel of turning.
When the prestage pressurizing function is carried out by the above-described vacuum servo apparatus, for example, in the case in which the vehicle is brought into the oversteered state in turning the vehicle, the actuator is driven with no braking operation of the driver and the rear chamber is communicated with the atmosphere by driving the actuator. A significant pressure difference is produced between the front chamber and the rear chamber to thereby produce an output. Successively, it is conceivable that the output from the vacuum servo apparatus is converted into hydraulic pressure by a well-known master cylinder and the pressurized brake fluid is supplied to the pump and the pump sucks the pressurized brake fluid and supplies the brake fluid to the outer side front wheel of turning.
However, according to the constitution of the hydraulic pipes of the brake device having the brake steering control, it is conceived that wheels other than the outer side front wheel of turning are constituted to communicate with the master cylinder. That is, according to the known vacuum servo apparatus, by driving the actuator, the atmosphere is completely delivered into the rear chamber and accordingly, strong output is produced and in the brake steering control, when the conventional vacuum servo apparatus is operated as the prestage pressurizing function, strong hydraulic pressure is directly provided from the master cylinder to wheels other than the wheel constituting the object of the brake steering control and there is a concern that the function of the brake steering control cannot be achieved sufficiently.
A need thus exists for a vacuum servo apparatus capable of reducing the variation in the input and output characteristics